Spread-spectrum communication system

ABSTRACT

A receiver in a spread-spectrum communication system is adapted to receive a signal which is modulated by a binary code modulated by a binary code or a data, and includes a matched filter which detects a correlation between the received signal and a reference signal modulated by a reference code produced by a code generator. When the received code supplied from the matched filter coincides with or is slightly displaced from the reference code, a pulse is produced from a correlation spike waveform with a relatively large amplitude to extract a desired pulse from the pulse and use it to initialize the reference code generator.

FIELD OF THE INVENTION

This invention relates to a digital wireless communication system, andmore particularly to a spread-spectrum communication system wherein thetransmitter modulates a carrier by a binary code modulated by data,instead of directly modulating the carrier by the data, so that thesignal arriving at a receiver's end is received via a matched filter.

BACKGROUND OF THE INVENTION

In such a spread-spectrum communication system, a carrier is modulatedby a pseudo-noise code (hereinafter called PN code) which is a binarycode and also is modulated by data, as shown in FIG. 1(A). In FIG. 1A,reference numeral 1 designates a data source, 2 is a modulator, 3 is aPN code generator, 4 is a carrier wave generator, 5 is a modulator, and6 is a transmitter. At a receiver's end, as shown in FIG. 1(B), amatched filter detects a correlation between the arriving signal and areference PN code therein so that when both the codes coincide or areslightly displaced, an autocorrelation waveform (hereinafter called"correlation spike waveform") with a relatively large amplitude isprocessed and demodulated into the data. In FIG. 1B, reference numeral 7refers to a receiving antenna, 8 is a correlator, 9 is a reference PNcode generator, 10 is a data demodulator, and 11 is a data stream.

A convolver is used as a matched filter. A convolver in general is afunctional unit to effect a convolution integral, but may be a matchedfilter to effect a correlation operation if a binary reference code(hereinafter called "reference code") is the time-reversed image of thereceived code.

A surface acoustic wave convolver (hereinafter called "SAW convolver")is referred to as one type of convolver. The industry provides differentstructures of SAW convolvers, i.e., a lamination of a piezoelectricsubstance and silicon spaced by a gap, a combination of a piezoelectricsubstance and silicon united via an oxide layer, a single body of apiezoelectric substance, etc. They all have nonlinear properties andperform a correlation operation of two signals by use of an interactiontherebetween, the result of which is integrated by a gate electrodeprovided in an interaction region.

FIG. 2 shows a construction of a SAW convolver. Reference numerals 12and 13 are transducers, 14 is a piezoelectric substance, 15 is an oxidelayer, 16 is a silicon substrate, and 17 is a gate electrode. A signals(t) entered from the transducer 12 travels to the right in the Figure,and a signal r(t) entered from the transducer 13 travels to the left.Since the construction comprising the piezoelectric layer, oxide layerand silicon has a nonlinear property, an interaction occurs between thesignals s(t) and r(t) to cause the correlation operation and integratethe result therefrom in the gate electrode 17.

A signal c(t) produced by the gate electrode 17 is expressed by:##EQU1##

Where A is a constant, T is the time required for an acoustic wave topass under the gate electrode (hereinafter called "under-gate delaytime"), x is the propagation distance of the signal s(t), and s is thesonic speed.

A PN code in general has a given cycle. In formation of a waveform atthe transmitter's end, it is often employed to correlate one cycle ofthe PN code with one bit length of the data. For a better explanation orunderstanding, the instant description takes an example where one cycleof the PN code equals the length of one data bit.

The relationship between the under-gate delay time and the PN code maybe selected as desired. Namely, the under-gate delay time may beshorter, equal or longer with respect to one cycle of the PN code. Theunder-gate delay time means the integral period in the correlationoperation. The integral period preferably equals one cycle of the PNcode, considering the correlation characteristics of the PN code. Theinstant description takes an example where the under-gate delay timeequals one cycle of the PN code.

The above-cited relationships are shown FIGS. 3A, 3B, and 3C. FIG. 3Ashows a data bit, and FIG. 3B shows an arrangement of the PN code. TheFIGS. 3A and 3B show that the length l of one data bit equals one cycleof the PN code. FIG. 3C is a diagrammatic cross-sectional view of aconvolver wherein the delay time in the length L of the gate electrodeequals l. Here again, the illustrated arrangement is simply an example,and any relationship may be selected between one data bit, one cycle ofthe PN code and the under-gate delay time.

In practical communication, the receiver always stands by for reception,with the reference signal entered in one of the transducers. When asignal is received, it is supplied from the other transducer to theconvolver. If the PN codes involved in the received signal coincideswith the reference signal, the gate electrode of the convolver providesa correlation spike waveform. However, it is still unknown in whichposition the both codes are aligned. The data is not demodulated intoits proper form unless the alignment is established at a properposition. For example, if both the codes are aligned at the positionshown by FIG. 4A, the received PN code is shared half and half by databits A and B. In FIGS. 4A and 4B, D shows a data bit, R shows thereceived PN code, RP shows the reference PN code, and L shows the regionunder the gate electrode where the interaction occurs. A is thetime-reversed PN code of the PN code A.

As described above, some means is necessary to finally make both thecodes coincide at the position of FIG. 4B if they first coincide in anyother position. The first coincidence of both the codes after receptionof the arriving signal and until the coincidence at the position of FIG.4B is called "primary synchronization" in this text. Although theprimary synchronization is out of the scope of the present invention, itis described in a report by D. Brodtkorb and J. E. Laynor entitled "Fastsynchronization in a spread-spectrum system based on acoustoelectricconvolvers" and printed in pages 561 through 566 of 1978 UltrasonicsSymposium Proceedings, IEEE Cat. No. 78CH1344-ISU.

After the primary synchronization is established, and the placement ofFIG. 4B is once obtained, a possible difference between the clockfrequency of the received PN code and the clock frequency of thereference PN code gradually displaces the alignment position from theplacement of FIG. 4B. The displacement at every instant of encounter ofthe heads of the received and reference PN codes is expressed by:##EQU2## where ƒr is the clock frequency of the reference PN code, ƒt isthe clock frequency of the received PN code, and N is the number ofchips in one cycle of the PN code.

Regardless of the primary synchronization, a possible difference betweenthe clock frequencies of both the codes gradually displaces thealignment position from the proper placement and disables demodulationof the data. This means that clock oscillators with an exactly uniformclock frequency must be provided in the transmitter and the receiver.Such clock oscillators are normally based on quartz oscillators.However, it is extremely difficult to manufacture a plurality of quartzoscillators with an exactly uniform frequency. Besides this, theyrequire a strict control of the environment such as temperature andhumidity.

The report by D. Brodtkorb et al. discloses how to overcome the problem.They use a counter to detect the displacement and shift time base of thereference PN code so as to cure the displacement. The misalignmentcorrection and subsequent maintenance of the perfect alignment at theproper position are called "synchronization maintenance" in this text.The proposal by D. Brodtkorb et al., however, has a drawback in that thesignal processing is complicated because it requires operation to driveor stop the counter.

OBJECT OF THE INVENTION

It is therefore an object of the invention to provide a spread-spectrumtransmitter/receiver system only requiring a simple signal processing toestablish an operable condition and not requiring any additionaloperation to drive or stop a counter, for example.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a receiver in aspread-spectrum communication system adapted to receive a signal whichis modulated by a binary code modulated by a binary code or data, andincluding a matched filter for detecting a correlation between thereceived signal and a signal modulated by a reference binary code, saidreceiver comprising:

first means for producing a correlation spike pulse from an correlationwaveform signal with a relatively large amplitude when said binary codein said received signal supplied from said matched filter coincides withor has a small displacement from said reference binary code;

second means for selectively extracting a desired correlation spikepulse from a plurality of said correlation spike pulses from said firstmeans and producing a reset pulse therefrom; and

a code generator for producing said reference binary code andinitialized by said reset pulse.

The invention will be better understood from the description givenbelow, referring to preferred embodiments illustrated in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1B and 1A are block diagrams of a transmitter and a receiverrespectively in a prior art spread-spectrum communication system;

FIG. 2 is a cross-sectional view of a convolver;

FIGS. 3A and 3B are timing element chart showing placements of a databit in FIG. 3A and a PN code in FIG. 3B related to a gate electrode of aconvolver in FIG. 3C;

FIGS. 4A and 4B are timing diagrams showing a misalignment and a properalignment of a received PN code and a reference PN code;

FIG. 5 is a block diagram of a receiver in a spread-spectrumcommunication system embodying the invention;

FIGS. 6A-6D are timing diagrams showing different alignment conditionsof the received PN code and the reference PN code;

FIGS. 7A and 7B are timing diagrams showing different alignmentconditions of the received PN code and the reference PN code; and

FIG. 8 is a block diagram of a modified part of the receiver of FIG. 5,using a two-gate convolver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment of the invention, a means employed to producea reset pulse from the above-referred correlation spike wave comprises:an amplifier which amplifies a signal having the correlation spikewaveform; an envelope detector which extracts an envelope signal in theamplified waveform; a threshold detector which compares the amplitude ofthe envelope signal with a predetermined threshold and produces anoutput pulse responsive to the result of the comparison; a wave shapingcircuit which shapes the output pulse from the threshold detector into apulse with a predetermined width; a gate pulse generating circuit whichproduces a gate pulse from a pulse synchronizing the reference PN code;and a gate circuit which selectively extracts the pulse from the waveshaping circuit responsively to the gate pulse from the gate pulsegenerating circuit. The matched filter is preferably a surface acousticwave (SAW) convolver. In second preferred embodiment of the invention,the binary code involved in the received signal and the signal modulatedby the reference binary code are both phase-modulated signals. Thebinary code may preferably be a PN code. In third preferred embodimentof the invention, if the matched filter is a two-gate convolver whichhas two gate electrodes provided on a single substrate, the envelopedetector extracts an envelope through a low pass filter after detectingphases of correlation spikes produced concurrently from both of the gateelectrodes.

FIG. 5 is a block diagram of a receiver in a spread-spectrumcommunication system embodying the invention. Assume now that thecarrier of a received signal 18 is two-phase modulated. The receivedsignal 18 is supplied to one of the transducers of a convolver 20 via animpedance matching circuit 19. A PN code produced by a reference PN codegenerator 27 two-phase modulates a sine wave produced by a carriergenerator 30 in a mixer 29. The modulated signal is supplied to theother transducer of the convolver 20 via an impedance matching circuit31 and serves as a reference signal 32. Thus, the primarysynchronization is established between the PN codes related to thereceived signal and the reference signal entered in the convolver 20,thus representing an alignment as shown in FIG. 4B, for examle. Hereassume that a difference exists between their clock frequencies thatgradually displaces the position of the alignment to a position shown inFIG. 6A where the upper waveform shows the received PN code PN₁, thelower waveform shows the reference PN code PN₂, and Δπ is a displacementbetween the alignment position of the two codes and an interactionregion under the gate electrode. A correlation spike waveform thenproduced by the gate electrode is entered in the amplifier 22 via theimpedance matching circuit 21 and is amplified therein. The envelopedetector 23 subsequently extracts an envelope of the spike. Thethreshold detector 24 compares the extracted envelope waveform with apredetermined threshold and produces a pulse when the extracted waveformis larger than the threshold. The wave shaping circuit 25 shapes thepulse into a pulse with a predetermined width. Here the pulse obtainedby the procession of the correlation spike waveform is known as a"correlation pulse". Since the received signal and the reference signaltravel in the opposite directions, they are aligned in two positionsshown in FIGS. 7A and 7B. Therefore, some arrangements of the dataprovide two correlation pulses in one cycle of the PN code. The gatecircuit 26, however, extracts only one of the correlation pulsesresponsive to the alignment with a less or smaller displacement as shownin FIG. 7A. The extraction is effected by detecting the correlationpulse and a gate pulse produced in the gate pulse generator 28. The gatepulse generator produces the gate pulse by processing a pulse which isgenerated by the reference PN code generator 27 and synchronizes the PNcode.

The extracted correlation pulse is used as a reset pulse to initializeall the shift registers constituting the reference PN code generator.Thus the reference PN code is started from the beginning. This is shownin FIG. 6B. In FIGS. 6B, 6C, and 6D, the upper waveforms show thereceived PN code PN₁ the lower waveforms show the reference PN code PN₂,y is the point where the reference PN code is started again, and z isthe leader of the reference PN code before initialization. The alignmentshown in FIG. 6C shows that the two codes are aligned after a point xdesignating that the head of the received PN code moves to the right byone cycle from the position in FIG. 6B.

At this time, the displacement from the proper position is reduced toabout a half. The displacement is further reduced to a quarter as shownin FIG. 6D by another similar operation, using the last obtainedcorrelation pulse.

With continuous, repeated performance of the displacement correctingoperation, the invention device can maintain the synchronization.

A two-gate convolver having two gates provided on a single substrate isoften used for reception of a signal whose carrier is phase-modulated bya code which is obtained by a DPSK modulation using data. The use of thetwo-gate convolver enables extraction of an envelope by a delayeddetection. The extracted envelope is processed in the same manner togenerate a reset pulse. FIG. 8 shows an example of the use of a two-gateconvolver. The reference signal s(t) is entered from the left hand ofthe two-gate convolver, and the received signal r(t) is entered from theright hand. Reference numeral 34 designates a phase detector, 35 is alow pass filter, 36 is a positive threshold detector, 37 is a negativethreshold detector, and 38 is a logic OR circuit which produces anoutput to be applied to the wave shaping circuit 25.

As described above, the invention does not use any counter, and reliablymaintains the synchronization by a simple signal processing.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defines as follows:
 1. A receiver in aspread-spectrum communication system adapted to receive a signal whichis .Iadd.first .Iaddend.modulated by .[.a.]. .Iadd.binary data andthereafter by a .Iaddend.binary code .[.modulated by a binary code ordata.]., and including a surface acoustic wave convolver for detecting acorrelation between the received signal and a signal modulated by a.[.time-reversed.]. reference binary code .Iadd.in the form of atime-reversed replica .Iaddend.of said binary code, said receivercomprising:first means for producing a correlation spike pulse from acorrelation waveform signal with a relatively large amplitude when saidbinary code in said received signal supplied .[.from.]. .Iadd.to.Iaddend.said surface acoustic wave convolver coincides with or has asmall displacement from said reference binary code; second means forselectively extracting a desired correlation spike pulse from saidcorrelation spike pulses from said first means and producing a resetpulse therefrom; and a code generator for producing said referencebinary code and initialized by said reset pulse, wherein said firstmeans include: an amplifier for amplifying said correlation waveformsignal; an envelope detector for extracting a signal having an envelopewaveform in an output signal from said amplifier; a threshold detectorfor comparing the amplitude of said envelope waveform signal with apredetermined threshold and generating a pulse responsive to the resultof the comparison; and wherein said second means include: a wave shapingcircuit for shaping said pulse from said threshold detector into a pulsewith a predetermined width; said reference binary code beingsynchronized by a synchronizing pulse; a gate pulse generator forgenerating a gate pulse from said pulse synchronizing said referencebinary code; and a gate circuit responsive to said gate pulse toselectively extract said pulse supplied from said wave shaping circuit..Iadd.
 2. A receiver in a spread-spectrum communication system adaptedto receive a signal which is first modulated by binary data andthereafter by a binary code, and including a surface acoustic waveconvolver for detecting a correlation between the received signal and asignal modulated by a reference binary code in the form of atime-reversed replica of said binary code, said receivercomprising:first means for producing a correlation spike pulse from acorrelation waveform signal with a relatively large amplitude when saidbinary code in said received signal supplied to said surface acousticwave convolver coincides with or has a small displacement from saidreference binary code; second means for selectively extracting a desiredcorrelation spike pulse from said correlation spike pulses from saidfirst means and producing a reset pulse therefrom; and a code generatorfor producing said reference binary code and initialized by said resetpulse. .Iaddend.